Donut Dog is a free app that is cute and fun to use. It is similar to apps like Forest and Flora but your rewards come largely in the form of baked goods.

Forest is a pomodoro app that aims at making focusing more engaging through the use of visuals on how long users focus for. For Forest, this comes in the form of seeing how many trees one has grown. However, there is not much fun in planting virtual trees all day and for this I recommend Focusbyte.